Internet DRAFT - draft-huque-dnssec-alg-nego
draft-huque-dnssec-alg-nego
Internet Engineering Task Force S. Huque
Internet-Draft Salesforce
Intended status: Standards Track H. Shulman
Expires: January 23, 2019 Fraunhofer Institute
S. Kerr
Oracle Dyn
July 22, 2018
Algorithm Negotiation in DNSSEC
draft-huque-dnssec-alg-nego-03
Abstract
This document specifies a DNS extension that allows a DNS client to
specify a list of DNSSEC algorithms, in preference order, that the
client desires to use. A DNS server upon receipt of this extension
can choose to selectively respond with DNSSEC signatures using the
most preferred algorithm they support. This mechanism may make it
easier for DNS zone operators to support signing zone data
simultaneously with multiple DNSSEC algorithms, without significantly
increasing the size of DNS responses. It will also allow an easier
way to transition to new algorithms while still retaining support for
older DNS validators that do not yet support the new algorithms.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 23, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
Huque, et al. Expires January 23, 2019 [Page 1]
Internet-Draft DNSSEC Algorithm Negotiation July 2018
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. DNSSEC Preferred Algorithms Option . . . . . . . . . . . . . 4
3. Why not use RFC 6975 for Algorithm Signaling? . . . . . . . . 4
4. Changes to Clients . . . . . . . . . . . . . . . . . . . . . 5
5. Changes to Servers . . . . . . . . . . . . . . . . . . . . . 5
6. Cache Considerations . . . . . . . . . . . . . . . . . . . . 6
7. Preventing Downgrade Attacks . . . . . . . . . . . . . . . . 6
8. Dealing with Proxies and Middleboxes . . . . . . . . . . . . 6
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
10. Security Considerations . . . . . . . . . . . . . . . . . . . 7
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
12.1. Normative References . . . . . . . . . . . . . . . . . . 7
12.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The DNS Security Extensions (DNSSEC) specifications [RFC4033]
[RFC4034] [RFC4035] support multiple signature algorithms. A DNS
zone can be signed simultaneously with multiple algorithms, but there
is no provision in the current specifications to negotiate the
selective delivery of signatures of a specific algorithm in DNS
responses.
In contrast, many other security protocols, like TLS, IKE, SSH and
others, support an algorithm or cipher suite negotiation mechanism to
enable the client and server to select the "best" algorithm they
jointly support.
This means that DNS servers have to send responses with signatures of
all algorithms that the requested data are signed with, which can
result in significantly large responses. Not only is this
inefficient in terms of the additional communication and processing
overhead, but it often causes a variety of operational problems.
Most DNS queries and responses utilize UDP transport today. While
Huque, et al. Expires January 23, 2019 [Page 2]
Internet-Draft DNSSEC Algorithm Negotiation July 2018
the EDNS0 specification can support very large DNS over UDP payload
sizes, once they exceed the common Internet Path MTU (typically about
1,500 octets), they need to be fragmented at the IP layer. Many
studies [add citations] have shown that IP fragmentation does not
work reliably on today's Internet, because fragments are often
blocked by network security devices. Furthermore, fragments can
cause a variety of security and operational issues, as documented in
[frag-bad], and previous attempts to deprecate fragments in the IETF.
Constraining DNS message sizes to sit comfortably within the
network's Path MTU can avoid these problems, and in fact modern UDP
Usage Guidelines [RFC8085] strongly recommend this.
DNS can run over other transports that can obviate the IP
fragmentation problem, such as TCP (with Path MTU discovery or a
suitably configured Maximum Segment Size) and TLS. In fact, some
operators are known to truncate a DNS payload in preference to
emitting a a response that is likely to be fragmented, instructing
the client to re-query over TCP. However, these alternative
transports have not been widely deployed in the field, and there is
some reluctance by operators to make wide use of TCP or TLS because
of their added processing and performing costs. This situation may
change over time, but at least today, the dominant transport for DNS
query and response remains UDP.
The response size issue is also a significant barrier to the
introduction of new algorithms in DNSSEC. As can be readily seen
from the RSA to ECDSA transition, very few zones have transitioned
from RSA to ECDSA, and furthermore, very few have been willing to
sign their zones with multiple algorithms. Newer DNSSEC algorithms
have already appeared or are being proposed: EdDSA [RFC8080], NSEC5
[nsec5], and it is expected that some time in the future, there will
be post quantum signature algorithms for DNSSEC.
It is often not feasible to deploy only a new algorithm before the
field of deployed validating resolvers have been updated to
understand it. This would cause the zone to appear unsigned to those
validators, negating the security that DNSSEC provides, and posing
critical security problems for applications like DANE [RFC6698]
[RFC7671] that rely on DNSSEC authentication. Thus new algorithms
will require zone operators to simultaneously deploy multiple
algorithms, and support older algorithms for an extended period of
time until the population of validators have upgraded themselves to
support the newer algorithms.
This document proposes a new mechanism by which a DNS client when
sending a query can indicate an ordered list of DNSSEC signature
algorithms it desires to use. The DNS server can use this
Huque, et al. Expires January 23, 2019 [Page 3]
Internet-Draft DNSSEC Algorithm Negotiation July 2018
information to selectively construct a response with only the
signatures using the most preferred algorithm that it supports.
2. DNSSEC Preferred Algorithms Option
The EDNS0 specification outlined in [RFC6891] defines a way to
include new options using a standardized mechanism. These options
are contained in the RDATA of the OPT meta resource record. This
document defines a new EDNS0 option called "DNSSEC Preferred
Algorithms" used by a client to indicate an ordered list of DNSSEC
signature algorithms that it supports and prefers. This option can
appear only once in an OPT RR.
0 8 16
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-CODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| LIST-LENGTH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ALG-CODE | ... |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
OPTION-CODE is the code (TBD) assigned by IANA for this EDNS0 option.
LIST-LENGTH is the length of the list of digital signatures or hash
algorithm codes in octets. Each algorithm code occupies a single
octet.
ALG-CODE is the list of assigned values of DNSSEC zone signature
algorithms that the client prefers to be used. The algorithms are
listed in the order preferred by the client.
3. Why not use RFC 6975 for Algorithm Signaling?
The new EDNS0 option described in this document is very similar to
the DNSSEC Algorithm Understood (DAU) option defined in [RFC6975]
(Signaling Cryptographic Algorithm Understanding). That
specification has not seen much adoption or even implementation, and
it has been suggested that it could be repurposed to implement the
algorithm negotiation mechanism described in this document.
This document proposes a new option instead, because the RFC6975
option could not be reused without significantly revising its
semantics. For example, it currently says that the list of algorithm
codes is unordered, and that the server must not infer any ordering
Huque, et al. Expires January 23, 2019 [Page 4]
Internet-Draft DNSSEC Algorithm Negotiation July 2018
or preferences from the list. Furthermore, it states that the option
must not trigger any special processing on the server side.
In addition to algorithm negotiation, this new option could also
subsume the additional function of signaling algorithm understanding
originally intended by RFC 6975.
4. Changes to Clients
A client is defined to be any DNS speaker that issues a query, e.g. a
stub resolver, a resolver issuing outbound queries to authoritative
servers, or to other resolvers etc.
A client implementing this specification and configured to use it,
adds an EDNS0 DNSSEC Preferred Algorithms option to the OPT Pseudo
Resource Record in the Additional Section of the query, listing its
desired DNSSEC algorithm numbers in preferred order. It only makes
sense to add this option if the client is requesting DNSSEC
signatures, so the DNSSEC-OK bit in the EDNS Flags field MUST also be
set.
As a general rule, to maximize security, the client should prefer
stronger DNSSEC algorithms to weaker ones.
5. Changes to Servers
A server is defined to be any DNS speaker that sends DNS responses,
e.g. an authoritative server, or a resolver when answering queries
from downstream clients.
Upon receipt of a query with the DNSSEC-OK bit set, and the DNSSEC
Preferred Algorithms EDNS0 option, an Authoritative Server SHOULD
include in its response, DNSSEC signatures using only the most
preferred algorithm that it supports. It also includes the Preferred
Algorithms EDNS0 option in the response, to indicate that it
recognizes the option and has acted on it.
If an Authoritative Server has no algorithms in common with the
Preferred Algorithms list in the incoming query, it MUST send back a
SERVFAIL response (Response Code 2). This response MUST contain the
list of algorithms supported by the server in the EDNS0 Preferred
Algorithms option. [Note: this specific behavior may be
controversial since it is a depature from plain DNSSEC's fail open
behavior. Alternative behavior could be specified, such as returning
a NOERROR response with no signatures.]
If a resolver receives a query from a downstream validating client
with a Preferred Algorithms list different from its own, then it
Huque, et al. Expires January 23, 2019 [Page 5]
Internet-Draft DNSSEC Algorithm Negotiation July 2018
should send outbound queries with the client's preferred list, and
return answers appropriately.
6. Cache Considerations
A Validating Resolver answering queries with the DNSSEC-OK bit set
from data in its cache needs to take a few additional steps. If the
query does not include the Preferred Algorithms option, and the
resolver has selectively cached signatures of a subset of algorithms
supported by the zone containing the query domain name, then it MUST
re-send outbound queries to the authoritative server without the
Preferred Algorithms option in order to retrieve the entire set of
signatures for the query. If the query includes the Preferred
Algorithms option, but prefers algorithms known to be supported for
the name, but different from what has been cached, the resolver MUST
again send outbound queries to retrieve answers with signatures the
client prefers, by copying the client's Preferred Algorithms option
into the outbound query.
7. Preventing Downgrade Attacks
There is no cryptographic integrity protection of EDNS0 options. In
theory, Transaction Signatures [RFC2845] could be deployed to
integrity protect the entire query message with per-client keys in
closed populations of DNS speakers, but this is not a viable
mechanism in the general case of arbitrary DNS clients and servers on
the Internet.
Hence an active man-in-the-middle attacker could strip out stronger
algorithms from the client's preferred algorithms list and force the
server to send back signatures with a weaker algorithm than it might
have otherwise sent.
In order to detect such attacks, the client MUST compare the zone
signing algorithms listed in the zone's authenticated DNSKEY RRset,
and the preferred list in the query that it sent, to the algorithms
seen in the response signatures. If signatures by the most preferred
algorithm they have in common have not been sent, this may indicate
an algorithm downgrade attack.
8. Dealing with Proxies and Middleboxes
EDNS is a hop-by-hop mechanism. Hence all DNS speakers in the path
from the querier invoking this option to the responding server need
to support this mechanism for it to work correctly. DNS proxies
along the path that transparently relay requests and responses, and
largely comply with the implementation guidelines described in
[RFC5625] should not be a problem. But more complicated proxies,
Huque, et al. Expires January 23, 2019 [Page 6]
Internet-Draft DNSSEC Algorithm Negotiation July 2018
middleboxes, forwarding resolvers, etc that actively interpret DNS
messages, but do not understand this new option, will likely strip
off the unrecognized option in their outbound queries. The result
will be that the responding server will send back signatures made
with the full set of algorithms.
There is always a danger that a misbehaving middlebox might block or
drop a DNS packet with an unrecognized EDNS option, but this is a
threat that applies to almost all DNS extension proposals.
Deployment of new DNS options provides an opportunity to identify and
remove or fix such misbehaving devices.
An alternative end-to-end mechanism is described in [dnssec-nego] to
workaround DNS speaking middleboxes that haven't been upgraded to
recognize this option. It involves the client encoding the ordered
list of algorithms in a sequence of labels prepended to the query
name, and the addition of a new DNSKEY RR (with a new algorithm
number) at the authoritative server to signal to clients that the
server recognizes these specially constructed query names. No
further details of this alternative mechanism are provided in this
document, but could be incorporated in future revisions if there is
interest in developing that solution.
9. Acknowledgements
This specification builds on earlier work on DNSSEC algorithm
negotiation by Amir Herzberg and Haya Shulman in [dnssec-nego].
10. Security Considerations
[ TODO ]
11. IANA Considerations
This specification requires the registration of a new value in the
DNS EDNS0 Option Code Registry, maintained by IANA.
12. References
12.1. Normative References
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.
Huque, et al. Expires January 23, 2019 [Page 7]
Internet-Draft DNSSEC Algorithm Negotiation July 2018
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<https://www.rfc-editor.org/info/rfc4034>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013, <https://www.rfc-
editor.org/info/rfc6891>.
12.2. Informative References
[dnssec-nego]
Herzberg, A. and H. Shulman, "Cipher-Suite Negotiation for
DNSSEC: Hop-by-Hop or End-to-End?", in IEEE Internet
Computing, February 2015,
<http://ieeexplore.ieee.org/document/7031814/>.
[frag-bad]
Herzberg, A. and H. Shulman, "Fragmentation Considered
Poisonous", in IEEE Conference on Communications and
Network Security, October 2013,
<http://ieeexplore.ieee.org/document/6682711/>.
[nsec5] Vcelak, J., Goldberg, S., Papadopoulos, D., Huque, S., and
D. Lawrence, "NSEC5, DNSSEC Authenticated Denial of
Existence", <https://tools.ietf.org/html/draft-vcelak-
nsec5>.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,
<https://www.rfc-editor.org/info/rfc2845>.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,
<https://www.rfc-editor.org/info/rfc5625>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <https://www.rfc-editor.org/info/rfc6698>.
Huque, et al. Expires January 23, 2019 [Page 8]
Internet-Draft DNSSEC Algorithm Negotiation July 2018
[RFC6975] Crocker, S. and S. Rose, "Signaling Cryptographic
Algorithm Understanding in DNS Security Extensions
(DNSSEC)", RFC 6975, DOI 10.17487/RFC6975, July 2013,
<https://www.rfc-editor.org/info/rfc6975>.
[RFC7671] Dukhovni, V. and W. Hardaker, "The DNS-Based
Authentication of Named Entities (DANE) Protocol: Updates
and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
October 2015, <https://www.rfc-editor.org/info/rfc7671>.
[RFC8080] Sury, O. and R. Edmonds, "Edwards-Curve Digital Security
Algorithm (EdDSA) for DNSSEC", RFC 8080,
DOI 10.17487/RFC8080, February 2017, <https://www.rfc-
editor.org/info/rfc8080>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
Authors' Addresses
Shumon Huque
Salesforce
Email: shuque@gmail.com
Haya Shulman
Fraunhofer Institute
Email: haya.shulman@gmail.com
Shane Kerr
Oracle Dyn
Email: shane@time-travellers.org
Huque, et al. Expires January 23, 2019 [Page 9]
